Illuminating system



April 1966 R. H. HENSLEIGH 3,246,133

ILLUMINATING SYSTEM Filed Jan. 20, 1964 IN VENTOR.

A TTORNE Y United States Patent 6 Claims. Cl. 240-24 This invention relates to illumination systems and has reference to lighting for panel dials and the like.

While it is essential that an aircraft pilot have visual access to the dial faces of various meters and navigational instruments, an excess of transient light Within the cockpit leads to a phenomenon commonly known as light blindness which is essentially the retardation of visual acuity for distant or faintly lighted objects external to the aircraft. In addition to the obvious need for glare suppression the problem involves the efficient distribution of illuminating light over the several dial faces so that information bearing rays may be provided with uniformity and without extraneous concentrations of light which serve no useful function. Part of the difliculty resides in the inverse square law of illumination from a point source. Without proper corrective measures as set forth herein, a light bulb positioned exteri-orly above an instrument dial and out of a pilots line of sight to achieve an indirect lighting effect tends to illuminate those surfaces nearest the bulb to a greater extent than more remote areas are illuminated; since the illumination thus provided .is approximately inversely proportional to the square of the distance from the filament of the bulb, uniform lighting can only be achieved by this method when the illuminated area is small in comparison to the distance from the bulb. The dimensional restrictions of aircraft panels offer little encouragement for incorporation of this type of lighting therein, and variations as great as m 1 (in foot lamberts) have been tolerated in the aircraft industry heretofore. In flood lit panels the proximity of a light bulb to the plane of the panel face further complicates the achievement of uniform lighting by conventional methods.

In order to. enhance the apparent uniformity of lighting at an instrument panel, and particularly in circular panels where information bearing indicia may be concentrated at the perimeter of the panel, multiple light sources including circumferentially spaced bulbs have been used to some advantage. Here again, however, lighting contrasts generally result in so called hotspot areas having many times the illumination intensity of others of the same dial face. The limitations of such a lighting system can best be demonstrated by its logical extension to an infinite number of circumferential lighting sources which would result in the equivalent of a single light source in the form of the illuminating ring providing an illumination distribution concentrated at the perimeter of the dial face and diminishing toward the center thereof. Since a radial dimension for indicia on a flat dial face cannot be avoided, uniform distribution of light on the indicia cannot be achieved; on instruments Where it is not practical to concentrate all information at the perimeter of a dial, illumination from a plurality of bulbs is even less effective.

The use of fluorescent dials and indicators in conjunction with an ultraviolet light source presents a similar problem which is accentuated by the unfavorable atmospheric attenuation of ultraviolet rays. Extreme violet filters for incandescent bulbs can also produce fluorescence in a dial face, but generally operate at such a low efficiency that necessary shielding and heat dissipation means for intense light sources add new complexities to such a system. Fluorescent paints illuminated by radio- 3,246,133 Patented Apr. 12, 1966 active materials incorporated in the paint itself, as have been used on the dial faces of wrist watches, are efficient for a fixed intensity of illumination but cannot be controlled and are of little value during twilight hours when dial illumination must be increased.

Accordingly, an object of the present invention is to provide means for evenly distributing light over the surface of an instrument dial.

Another object of the invention is to provide a single source lighting system for an instrument dial face and wherein so called hot spots and transient light are minimized.

A further object of the invention is to provide a simple system for effectively flood lighting the various instruments of a control panel.

A particular object of the invention is-to provide a system of uniform illumination for instrument dial faces in aircraft wherein the intensity of illumination of such a lighting system may be controlled by the pilot.

Another object of the invention is to provide a new and useful optical system to effectuate even distribution of light over an illuminated surface.

A further object of the invention is to provide an optical system for the uniform distribution of light over an illuminated surface wherein the optical elements of such a system are susceptible to fabrication by molding techniques.

An important object of the present invention is to provide a system for more effectively spreading illumination over the surface of an instrument dial or the like and wherein the space requirements of such a system are comparable to the space requirements of systems known and used heretofore.

These and other objects will become apparent from the following description and the accompanying drawings wherein:

FIG. 1 is a perspective view of a simple cone and its complement as utilized in one form of the present invention but shown with the dimension of the axis of the cone greatly exaggerated for purposes of description.

FIG. 2 is a perspective view of a cylindrical section cut from the components illustrated in FIG. 1, and it also shows the axial dimensions of the cone and cylinder greatly exaggerated.

FIG. 2a is a perspective veiw of optical elements illustrated in FIG. 2.

FIG. 3 is a parallelepiped cut from a sector of the components illustrated in FIG. 1 and likewise having the axial dimensions of the cone thereof greatly exaggerated.

FIG. 4 is a perspective view of an instrument panel illustrating features of construction of a preferred form of the present invention.

An understanding of the working principles of total internal reflection of light rays within a transparent medium is essential to an understanding of the construction and operation of the present invention. When a ray of light passing through air strikes a plane surface of a transparent medium, part of the ray is reflected from the surface and part of the ray is refracted into the transparent medium. The incident reflected and refracted rays all lie in a common plane which is perpendicular to the surface of the medium in question. The normal is defined as a line peupendicular to the surface at a point where the ray strikes a transparent medium. As measured from the normal, the angle of reflection equals the angle of incidence. The angle of refraction, as measured from the normal, is defined by Snells law as sin I/ sin R=U where I is the angle of incidence, R is the angle of refraction, and U is the index of refraction of the medium in question. Except for the rays perpendicular to the surface, the angle of incidence is always greater than the "ice angle of refraction. For ordinary glass which has a refractive index of about 1.5 a ray at grazing incidence is refracted into the glass at an angle of about 40 degrees. Reciprocally, a ray passing through the glass at an angle Of 40 degrees from a normal will emerge into air at grazing incidence to the surface of the glass; this angle is termed the critical angle for glass or any other transparent medium having the same refractive index. If a rayv passing through the medium in question should strike the surface at an angle greater than 40 degrees with respect to the normal it is obvious that the laws of refraction can no longer obtain, and the ray is totally reflected from the surface back into the medium. Depending upon its refractive index, there is a critical angle for every transparent medium; rays originating within the medium and striking a plane surface at an angle with respect to the normal greater than the critical angle are totally reflected within a medium.

The present invention utilizes the phenomenon of total internal reflection within parts of cones and cylinders, having complementary conical cavities, to achieve an even distribution of light over a surface to be illuminated and to capture certain transient rays and divert the same from the line of sightof an observer of a surface so illuminated. With particular reference to FIG. 1, a right circular cone of transparent material is generally designated by the numeral a cylinder 11 having a conical cavity 12 complementary to the cone 10 is spaced from and positioned above the cone 10 in coaxial relationship therewith. Only for purposes of the present description the axial dimensions of the cone 10 and cylinder 11 are shown in greatly exaggerated form, it being understood that in actual construction of the invention the altitude of the cone and corresponding length of the cylinder are very small in comparison to the diameters of these two bodies. On the axis 13 of the cone 10 any suitable light source 14 is placed Within the cone and may consist of a bulb imbedded therein or extended into the cone through an axial channel 15. If a ray emerges from the light source 14 along the axis of the cone toward the cylinder it will emerge from the apex of the cone 10 and traverse theairspace between the cone 10 and cylinder 11 and strike the cylinder at the apex of its conical cavity from which it will travel in a straight line to the upper surface of the cylinder. If an opaque or reflecting disc is placed upon the upper surface of the cylinder the path of the previously described axial ray will be blocked. If the disc 16 be placed concentrically with the axis of the cylinder and if the diameter of the disc be made great enough so that all rays emerging from the light source 14 and striking the upper surface of the upper cone 10 at an angle lessthan the critical angle are likewise blocked then no unreflected rays can emerge from the upper surface of the cylinder. Thus a ray 17 striking the upper surface of the cone 10 at less than the critical angle is intercepted by thedisc 16. A ray 18 emerging from the light source 14 and striking the upper surface of the cone 10 at an angle greater than the critical angle is totally and internally reflected back into the cone 10 and proceeds to the lower surface of the cone. The angle at which the;

ray 18 strikes the lower surface of the cone 10 with respect to the normal is not the same as the angle formed by the emerging ray and its reflection with the upper surface of the cone; the reason for the differences in angularity of successive reflection with respect to their normals within the cone is the fixed angle formed by the upper and lower surfacesof the cone 10 as viewed from any ray originating at the axis of the cone This angle (theta in FIG. 1) cause-s each internal reflection to form an angle with its normal less than that of its previous reflection. Since the original reflection of the last described ray 18 was only slightly greater than the critical angle, the second surface contact at the base of the cone results in a refraction of the ray to the surface beneath the cone. If the surface beneath the cone is not optically' smooth, the

ray 18 so refracted from the cone 10 will be scattered from the illuminated surface and part of the scattered ray will pass vertically from theilluminated surface through the cone and cylinder in the line of sight of an observer. A ray 19 which encounters several internal reflections, each at a lesser angle with respect to its normal than that of the preceding reflection, before emerging from the base of the cone will also be scattered upon Contact with the illuminated surface and some of its rays will likewise pass through the cone 10 and cylinder 11 bearing information to the line of sight of an observer.

Obviously, for successive internal reflections at dimin ishin-g angles with the normals of the reflecting surfaces, not all refracted rays will emerge from the lower surface of the cone 10; an equal number of rays will strike the upper surface of the cone 10 at angles less than the critical angle after two or more internal reflections. Consider a ray 20 which, after emerging from the light source 14 is totally reflected once by the upper surface of the cone 10 and once by the lower surface thereof and then approaches the upper surface of the cone at an angle less than the critical angle. Since incident, reflected, and refracted rays lie in a common plane and since the plane of the rays 20 includes the axis of the cone 10 and cylinder 11, the upper surface of the cone 10 where the ray 20 emerges and the surface of the conical cavity of the cylinder 11 which the'ray 20 enters may be considered as parallel lines in the plane of the ray 20. Equal angles are thus formed by the ray 20 with respect to its point of departure from the cone 10 and point of entry in the cylinder 11 and equal refractions occur; thus, the last line of travel of the ray 20 to the cone 10 is parallel with the first line of travel of the ray into the cylinder 11 and the angle formed by the ray 20 with the upper surface of the cylinder 11 is equal to the angle formed by the ray at its last reflection from the lower surface of the cone 10. But we have already seen that the last contact of the ray 20 with the lower surface of the cone 10 was at an angle of total internal reflection; therefore, the first contact with the upper surface of the cylinder 11 of the ray 20 also results in total internal reflection. Whereas succeeding reflections within the cone 10 are made at diminishing angles with respect to their normals, the converse is true of succeeding reflections within the cylinder 11; each internal reflection within the cylinder 11 is made at an angle with respect to its normal greater than that of the preceding reflection so that a ray once internally re.- flected in that body will proceed by one or more internal reflections to the cylindrical surface of the cylinder 11.

If a light absorbing material 21 (shown in fragment in FIG. 1) is placed about the cylindrical surface of the cylinder 11 nearly all unused rays refracted from the upper surface of the cone. 10 will be absorbed therein.

Thus, the only light to reach the eye of an observer is that scattered from the surface beneath the cone 10. The uniformity of distribution of light impinging upon a surface-beneath the cone 10 results from the fact that emergence of a ray from the lower surface of the cone 10 is dependent not upon distance from the light source 14 but upon the passage of successive reflections through a critical angle; it is easily deduced that no part of the lower surface of the cone 10 is favored in this respect. Misplacement of the bulb 10 can result in a non-uniform distribution of light beneath the cone 10 and more particularly can lead to a system of concentric rings representing areas of more intense illumination. This condition can be corrected, however, by adjustment of the position of the light source 14 within the cone 10 and by diffusion of light near its source.

The formation of complementary elements of a panel illumination system is illustrated in' FIG. 2. In this construction a cylindrical section is taken from the cone 10 and complementary cylinder 11 with the axis of the cone taken as a line on the surface of the sectioned cylinder 22. As shown by dotted lines complementary elements 23 and 24 together form the sectioned cylinder 22 are taken respectively as sections of the cone and cylinder 11, and the elements 23 and 24 are molded or otherwise fabricated and the light source is placed or focused in proximity to the line 25 representing the axis of the cone by which the elements are geometrically generated. FIG. 3 illustrates the geometry of a corner lit square or rectangular panel and the lower and upper elements 23a and 24a, taken from a sector of the cone 10 and cylinder 11, together form a parallelepiped; in this case, one edge 25a of the lower element 23a of the parallelepiped corresponds to the axis of the generating cone. Slight deviations from true conical surfaces for the cone 10 and cylinder 11 may be made without greatly impairing the efficiency of the system and may be found to be advantageous for special illuminating effects.

FIG. 4 illustrates in greater detail an application of the preferred form of the invention in the illumination of one dial face in an instrument panel. The lower element 23 of a circular section cut from a cone 10 is spaced from its complementary element 25 by an annular spacer plate 26 positioned between the optical elements. At the axis of the conical element 23 a radial saw-tooth disperser 27 is integrally constructed with the conical element; the several teeth of the disperser are each radially disposed with respect to the filament of an adjacent incandescent bulb 30, and the teeth serve to conduct and distribute light from the bulb into the conical element. With respect to a near point source of light, a plane interface between two conducting media acts as a convex lens slightly converging rays passing into the medium of greater optical density; a concavity at the area of the incandescent bulb can be used to eliminate this convergence, but it has been found that a radial saw-tooth disperser, as described and shown, effectively accomplishes the same result with better utilization of panel space. The perimeter of both optical elements 23 and 24, except so much of the perimeter of the conical element 23 as receives light directly from the bulb 30, are covered with a light absorbing material 31 such as black felt or with a coating of black paint. A metal perimetal frame 32 holds the optical elements 23 and 24 in proper orientation with respect to one another against the annular spacer plate 26 positioned between the optical elements interiorly adjacent the circumference thereof. A spacer ring 33 is received and enclosed by the perimetal frame 32 adjacntly beneath the circumference of the conical element 23 and is there held by the perimetal frame against an instrument dial face 34. By conventional attachment means the perimetal frame 32 is secured to the instrument to be illuminated, and either the instrument or the perimetal frame 32 or both may be mounted in and secured to a control panel 35. Rheostat 36 shown schematically in FIGURE 4, may be remotely positioned to control one or more light bulbs of the panel 35.

While operation of the illustrated embodiment of the invention has been discussed in the foregoing description, variations of the construction and adaptations for other applications will be suggested to those skilled in the art by further consideration of the operating principles of the invention. Escape of rays from the base of the cone 10 can be accelerated and concentrated at any part thereof by locally changing the angle of the cone. Variation of the rate at which the cone grows thinner toward its perimeter can be used to increase the intensity of light escaping from chosen portions of the base of the cone and transient light will be captured so long as the con cavity of the complementary element equals or exceeds the slope of corresponding parts of the cone so modified. In some applications, the light absorbed at the perimeter of the complementary element 24 in the foregoing description may be utilized to flood light areas surrounding the instrument face. In an edge lit panel the combined elements 10 and 11 would be inverted so that the base of the cone would face the observer; the invention could then be used with a replaceable or movable photonegative covering the base of the cone to illuminate printed matter such as a check list.

The invention is not limited to the exemplary construction herein shown and described but may be made in various ways within the scope of the appended claims.

What is claimed is:

1. An illuminating system comprising: a transparent conical optical element, a complementary transparent optical element having a conical cavity therein spaced from and coaxially disposed with said conical element, and a light source so positioned as to cause rays of light to radiate through said conical optical element from the vicinity of the axis thereof.

2. The invention as defined in claim 1 and wherein the common axis of said conical element and said cavity is located on corresponding parts of cylindrical surfaces comprising the circumferential sides of said conical element and complementary element respectively.

3. The invention as defined in claim 1 and wherein the common axis of said conical element and said cavity is located on corresponding parts of polygonal surfaces comprising the circumferential sides of said conical element and said complementary element respectively.

4. An illuminating system comprising a first optical element constructed of light conducting material and havingan axis and a circumference and a thickness diminishing conically from said axis to said circumference, a complementary element constructed of light conducting material and having an axis coline'ar with said axis of said first element and a circumference corresponding to said circumference of said first element having a thickness increasing from said axis of said complementary element to said circumference of said complementary element, and a light source so positioned as to cause rays of light to radiate through said first optical element from the vicinity of the axis thereof.

5. The invention as defined in claim 4 and including a light absorber on the circumference of said complementary element and means spacing said conical element from said complementary element.

6. An illuminating system comprising: an optical element of transparent material having one surface thereof corresponding to a part of the surface of a right circular cone with said part of said surface including the vicinity of the axis of said cone, a complementary optical element of transparent material having one surface thereof in the form of a cavity corresponding to a part of the surface of a right circular cone with said part of said surface including the vicinity of the axis of said cone, and a light source so positioned as to cause rays of light to radiate through the first said optical element from the vicinity of the axis thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,537,971 1/1951 Dames. 2,638,554 5/1953 Bartow et al 25099 2,740,957 4/1956 Davies 240-1 2,761,056 8/1956 Lazo 240-1 X 2,827,557 3/1958 Neugass 2401 X 2,831,282 4/1958 H'ardesty 2402.1 X 3,029,334 4/ 1962 Anderson et a1 L2401 3,040,168 6/1962 Stearns.

NORTON ANSHER, Primary Examiner. 

1. AN ILLUMINATING SYSTEM COMPRISING: A TRANSPARENT CONICAL OPTICAL ELEMENT, A COMPLEMENTARY TRANSPARENT OPTICAL ELEMENT HAVING A CONICAL CAVITY THEREIN SPACED FROM AND COAXIALLY DISPOSED WITH SAID CONICAL ELEMENT, AND A LIGHT SOURCE SO POSITIONED AS TO CAUSE RAYS OF LIGHT TO RADIATE THROUGH SAID CONICAL OPTICAL ELEMENT FROM THE VICINITY OF THE AXIS THEREOF. 